Impurity detection device and method

ABSTRACT

There is provided a sample preparation device and method for preparing a sample of liquid for detection of impurities. First ( 40 ) and second ( 38 ) electrodes are provided, located for immersion in a liquid under test. A semipermeable membrane ( 42 ) is positioned to protect the first electrode ( 40 ) from a body of liquid under test ( 32 ). The semipermeable membrane allows the liquid under test to pass therethrough to reach the first electrode, while preventing solids carried in the liquid from reaching the first electrode, the first electrode being positioned to affect the liquid under test in the vicinity of a sensor ( 36 ). Particular embodiments feature a hydrophilic membrane to protect the electrodes from suspended solids in the sample, a thin electrode assembly to achieve a faster response and the addition of a heater for temperature control to achieve consistent detection conditions and improved anti-fouling properties.

This patent application is a Division of prior U.S. patent applicationSer. No. 10/355,271 filed Jan. 31, 2003, now U.S. Pat. No. 7,201,831issued Apr. 10, 2007, and further is a Division of prior, U.S. patentapplication Ser. No. 11/657,510 filed Jan. 24, 2007, now U.S. Pat. No.8,080,831 issued Dec. 20, 2011, which U.S. patent application Ser. No.10/355,271 and U.S. patent application Ser. No. 11/657,510 claimedpriority under 35 U.S.C. 119 and 37 C.F.R. 1.55 based on PriorityDocument 0204186.1 filed in Japan on Feb. 22, 2002 and Priority Document0214367.5 filed in Japan on Jun. 21, 2002 and further claimed priorityunder 35 U.S.C. 119 based on prior foreign applications 0204186.1 filedin Great Britain on Jan. 22, 2002 and 0214367.5 filed in Great Britainon Jun. 21, 2002.

The present invention relates to methods and apparatus for detecting andmeasuring gases dissolved in liquids, particularly gases dissolved inliquids which return to their gaseous state under certain conditions oftemperature and pH. More particularly, the present invention relates tomethods and apparatus for preparing a sample of liquid for suchdetection and measuring.

The present application may be applied, for example, to the detectionand measuring of to the concentration of ammonium ions (NH₄ ⁺) andammonia molecules (NH₃) in water. The present invention may also beapplied to the measurement of chlorine (Cl) or cyanide ions (CN⁻) inwater, or any example where a gas may dissolve in a fluid but bereleased in its gaseous state under certain conditions of temperatureand pH.

PRIOR ART

FIG. 1 schematically shows a water quality measuring apparatus asdescribed in UK patent application GB-A-2290617. The illustrated deviceallows detection of dissolved species, such as chlorine or ozone inrelatively clean water such as tap water. The sensor device comprises aninsulating substrate 1 e.g. of ceramic, carrying interdigited electrodes2, 3, a pair of counter-electrodes 4, 5, a reference electrode 6 andconductive pads 8-12 to connect the device to electrical measurementapparatus. The reference electrode 6 may be partly surrounded by ascreen 7. The electrodes are typically of gold, or are gold plated. Inuse, a potential applied between electrodes 3 and 5 produces a currentrelated to chlorine concentration, and a potential applied betweenelectrode 2 and generator electrode 4 controls pH in the region ofworking electrode 3.

An example of a material that often needs to be measured is ammonia,which dissolves and dissociates in water according to the followingreaction:NH₃+H₂O⇄NH₄ ⁺+OH⁻

At pH values below 8.0 the equilibrium is well over to the right.However, at higher values of pH (e.g. pH around 11), the equilibriummoves over to the left. In order to detect the ammonium ionconcentration, it is possible to use a detector for NH₄ ⁺ ions. However,such detectors can be unreliable, because they may also detect, forexample, K⁺ and Na⁺ ions. Another possibility is to generate ammoniagas, by adding a suitable reagent containing hydroxyl ions (e.g. sodiumhydroxide) to the liquid. The ammonia gas can then be detected using anammonia gas detector.

Another material that it is often desirable to measure is chlorine,which dissolves and dissociates in water according to the followingreactions:Cl₂+H₂O⇄HOCl+HClHOCl⇄H⁺+OCl⁻HCl⇄H⁺+Cl⁻.

In EP-B-0637381 there is disclosed an ammonia gas sensor which is housedwithin a container partially immersed in a solution containing ammoniumions. An electrochemical generator is provided to generate hydroxyl ionsin a region of the solution adjacent the container. This convertsammonium ions to ammonia gas, which is sensed by the sensor after havingdiffused into a gas permeable membrane. This sensing provides anindication of the ammonium ions in the solution. This sensor is usefulin a number of applications including testing for contamination inwater. The addition of hydroxyl ions is achieved by means of theelectrochemical generator, which generates hydroxyl groups according tothe following reaction:2H₂O+2e ⁻→2OH⁻+H₂↑ or2H₂O+O₂+4e ⁻→4OH⁻.

WO-A-9625662 discloses a similar system. It also discloses apparatus fordetecting chlorine levels, by generating hydrogen ions with anelectrochemical generator, according to the following reaction:2H₂O−4e ⁻⇄O₂+4H⁺.

These documents disclose a method of detecting a material by increasingor decreasing the pH of the solution by electrochemical means. Thus, itis no longer necessary to add a reagent to the solution to increase ordecrease the pH. However, there are problems with these methods. Inparticular, the liquid being analysed may be subjected to stirring, or,when there is a liquid flow, to flow variations caused by turbulence.This has the effect that the OH⁻ ions generated at the cathode can beneutralised by H⁺ ions generated at the anode.

Another problem is that in certain liquids the different concentrationsof ions cause solids to precipitate upon the electrodes. A furtherproblem is that, in certain liquids, stagnation encourages bacterialgrowth, or biological contamination, of the sensor by the accumulationof biofilms.

According to WO98/22813, there is provided a pH altering device asillustrated in FIG. 2. The device 110 comprises a receptacle 112 havingliquid inlets 114 and 116 and liquid outlets 118 and 120. An interior122 of the receptacle is divided into first and second chambers 122 aand 122 b by a microporous membrane 124 which extends across theinterior 122. Each of the first and second chambers contains arespective electrode 126, 128. The microporous membrane 124 is arrangedsuch that the chamber 122 a is in fluid communication with the inlet 114and the outlet 118, but is not in fluid communication with the inlet 116and the outlet 120, and such that the chamber 122 b is in fluidcommunication with the inlet 116 and the outlet 120, but is not in fluidcommunication with the inlet 114 and the outlet 118.

The electrodes 126 and 128 are electrically connected to an electricalpower source 130 by wires 132 and 134.

The liquid to be analysed is flowed from pipe 136 to the flow splitter138, where it is split into two separate streams, each of which is fedto a respective one of the inlets 114 and 116. The feed pipe 136 may bein communication with any suitable source of liquid, such as, forexample, a river.

When a potential difference is applied across the electrodes 126 and 128an electrical current flows through the liquid by virtue of the abilityof anions and cations in the liquid to pass through the microporousmembrane 124.

The liquid stream fed to the chamber 122 a comes into contact with thecathode 126. Upon the application of a potential difference from thesource 130, hydroxyl ions are generated at the cathode 126. Thegeneration of the hydroxyl ions increases the pH of the liquid in thechamber 122 a and renders it relatively alkaline.

The liquid stream fed to the chamber 122 b comes into contact with theanode 128. Upon the application of a potential difference from thesource 130, hydrogen ions are generated at the anode 128. The generationof these hydrogen ions reduces the pH of the liquid in the chamber 122 band renders it relatively acidic.

The liquid stream in chamber 122 a is discharged from the receptacle 112via the outlet 118, and the liquid stream in the chamber 122 b isdischarged from the receptacle 112 via the outlet 120. The liquid streamdischarged through the outlet 118 will be relatively alkaline comparedwith the liquid stream discharged through the outlet 120. If the liquidsupplied to the device 110 from the pipe 136 were approximately neutral,then the pH of the liquid stream discharged via the outlet 118 would begreater than 7, and the pH of the liquid stream discharged via theoutlet 120 would be less than 7.

A particular use of the device of FIG. 2 relates to fluid analysis.Broadly, this involves electrochemically modifying a region of the fluidto convert a material to be measured into a form in which itsconcentration can be measured (such as a gas), and sensing the amount ofsaid form that is generated in order to provide a measure of theconcentration of the material in the fluid. In this application themicroporous membrane restricts or prevents any neutralisation caused byturbulence or mixing, so that an accurate measurement of theconcentration can be obtained.

In order to use the device in chemical analysis, it is preferred toprovide a sensing means in or downstream of the first and/or secondchambers. In order to perform chemical analysis it is important for oneor both electrodes to be capable of electrochemically modifying thefluid to convert a material of interest into a form in which it can bemeasured by the sensing means.

The sensing means may be adapted to sense ions in the solution (e.g.hypochlorite ions), or it may be adapted to sense a gas formed by theelectrochemical modification (e.g. ammonia, carbon dioxide or sulphurdioxide gas).

When the sensor is a gas sensor it may include a membrane permeable tosaid gas, and detection means for detecting the amount of said gas thatdiffuses into the membrane. The gas-permeable membrane of the sensor maybe any membrane that will permit the diffusion therethrough of thegaseous form of said material, but will prevent the passage of thefluid. The membrane may contain a dye sensitive to said gas, so that theconcentration of the material can be measured by measuring the magnitudeof the colour change of the dye. Means can be provided to measure themagnitude of the colour change.

The present invention is particularly applicable to the detection ofspecies such as ammonia in very dirty water such as sewage or industrialeffluent. The known devices suffer various problems in operating in suchconditions. For example, the devices of FIGS. 1 and 2 would very quicklybe fouled with solids to such an extent that the electrodes could nolonger act on the liquid. The concentrations to be measured may berelatively low, and a flow-through device such as that of FIG. 2 wouldneed to shift the pH of a large volume of water by a relatively largeamount in order to release a sufficient quantity of ammonia to bemeasured. Furthermore, the solubility of ammonia varies considerablywith temperature. Devices such as those of FIGS. 1, 2 would suffer fromvariation of temperature of the incoming fluid.

DESCRIPTION OF THE INVENTION

The present invention aims to alleviate at least one of the problemsencountered with the devices referenced above. The present inventionrelates particularly to a device and method for preparing a sample of aliquid for detection of a dissolved species which reverts to its gaseousform under predetermined conditions of temperature and pH.

In particular, the present invention provides a device for detection ofimpurities in an impure liquid, comprising a first electrode and asecond electrode located for immersion in a liquid under test; asemipermeable membrane positioned to delimit a sample volume from a bodyof the liquid under test, said semipermeable membrane allowing theliquid under test to pass therethrough to reach the sample volume, whilepreventing solids carried in the liquid from reaching the sample volume;and a sensor comprising a sensing element accessible to the samplevolume. An integrated element, comprising a porous or permeablesubstrate carrying the first electrode and a heater, is provided withinthe sample volume.

The semipermeable membrane may also prevent the solids from reaching thesecond electrode.

The integrated element may further carry the second electrode.

The integrated element may itself comprise: a porous or permeablesubstrate; the first electrode formed on a first side of the substrate;and the second electrode formed on a second side of the substrate. Theintegrated element may further comprise a heater element located betweenone of the sides of the substrate and the corresponding electrode, theheater element being insulated from the corresponding electrode. Theelectrodes and the heater element may be formed by deposition ofrespective conductive layers onto the substrate.

The first and second electrodes may be planar and concentric, the firstelectrode being substantially circular, the second electrode beingsubstantially annular, having an inside diameter larger than thediameter of the first electrode.

A gas volume may be provided between the first electrode and the sensor.

A gas permeable membrane may be provided to prevent the liquid in thesample volume from entering the gas volume, while allowing any gasemanating from the liquid to enter the gas volume.

A physical barrier may be provided between the first and secondelectrodes. The physical barrier may be porous or permeable to theliquid under test.

The device may further comprise a barrier around the first electrode,for restricting the movement of the liquid under test.

The device may further comprise a second heater for preventingcondensation of vapour generated from the liquid under test.

The device may be arranged for use with an ion sensitive electrode asthe sensor; alternatively arranged for use with a selective gas detectoras the sensor.

The semipermeable membrane may be hydrophilic, at least on the sidewhich is directed away from the sample volume. The gas permeablemembrane may be hydrophobic.

The device may further comprise means for measuring and controlling atleast one of the following characteristics of the liquid under test inthe region of the first electrode: pH, temperature, conductivity.

The device may further comprise an outer housing to contain theelectrodes, substrate, heater, membrane and sensor.

The present invention also provides a method of preparing a sample ofliquid for detection of impurities, comprising the steps of: providingfirst and second electrodes within the liquid; applying a voltagebetween the first and second electrodes, to thereby create a region ofincreased pH and a region of reduced pH at the respective cathode andanode electrodes; and detecting a characteristic of the liquid in afirst of the regions of changed pH. The method further comprises thesteps of: passing liquid from a body of liquid under test through asemipermeable membrane to reach the first region, thereby preventingsolids carried in the body of liquid from reaching the first region,while allowing the impurity to be detected to reach the first region;and heating the liquid in the first region of changed pH to a constanttemperature.

The temperature of the liquid in the first region may be adjusted topromote detection of the impurity to be detected.

The temperature of the liquid in the first region may be adjusted todeter the formation of biofouling on the membrane or on the electrodes.

-   -   The method may further comprise, in response to the alteration        of the pH of the first region, converting the impurity to be        detected into a gaseous form, and detecting the impurity with a        suitable gas detector.    -   The present invention also provides, in isolation, an integrated        element as defined above and having the first electrode formed        on a first side of the substrate, with the second electrode        formed on a second side of the substrate.

The above, and further, objects, advantages and characteristics of thepresent invention will become more apparent by consideration of thefollowing description of certain embodiments thereof, in combinationwith the accompanying drawings, wherein:

FIG. 1 shows a sensor device of the prior art for measuring dissolvedspecies under controlled conditions of pH;

FIG. 2 shows apparatus for preparing separated solutions of controlledpH by electrolysis across a semipermeable membrane;

FIG. 3 shows a schematic representation of a sample preparation deviceof the present invention;

FIGS. 4A-4E illustrate progressive stages in the manufacture of anintegrated element according to an embodiment of the present invention;

FIG. 5 shows a cross section through an integrated element of FIG. 4;

FIG. 6 shows a sample preparation and measurement device according to anembodiment of the present invention; and

FIG. 7 shows another view of the device of FIG. 6.

Problems encountered with known devices include the fouling of thesensors and/or membranes, particularly when dealing with the detectionof chemical species in waste water. Problems were also encountered inbuilding up enough pH differential, or achieving or maintaining therequired temperatures. Since the concentrations to be detected are low,there are problems in collecting a sufficient quantity of the species tobe detected to enable reliable detection. This is particularly true ofsensors in which the sample fluid may circulate, since the required pHwill not easily be achieved, and the temperature will not stabilise.

The present invention therefore aims to relieve at least some of theseproblems, as will be discussed with reference to FIG. 3, whichschematically represents a sample preparation device according to anaspect of the present invention.

According to the present invention, an impurity detection device 30 isprovided, to collect a sample 46 of a fluid 32 to be measured, and togenerate a gas sample 48 of the species under investigation. A suitabledetector 36 is required, although this could be of any known form, andthe construction and operation of the detector 36 itself does not formpart of the present invention.

The present invention may be applied to detect any species which issoluble in a liquid, but which returns to a gaseous state under certainconditions of temperature and pH. A common example is ammonia, whichoften needs to be detected in waste water and at water treatment plants.In the following description, the detection of ammonia in water will bedescribed, but this is in no way limiting of the scope of the invention.

A pair of electrodes 38, 40, current or voltage controlled, are used tolocally (electrochemically) adjust the pH of the sample water. Near theelectrode 40, the pH is changed beyond the value required to convert themeasurand species to a more volatile form (in this example, to convertammonium NH₄ ⁺ to ammonia NH₃↑), which evaporates and then is detectedas a gas by detector 36, which may be a simpler method of detection formany species. The pH shifted region near the electrode 40 could becomeacidic or alkaline according to the species to be detected, andaccording to the polarity of the applied current or voltage. Duringoperation, polarity of the electrodes could be reversed to perform acleaning and/or auto-zeroing action.

For the detection of ammonia, the electrode 40 is used as the cathode,to raise the pH of the surrounding liquid, thereby rendering it morealkaline, to release ammonia gas:NH₄ ⁺+OH⁻→NH₃↑+H₂O.

A semi-permeable membrane 42 is provided to protect the electrodes 38,40 from fouling by solids, and to reduce or eliminate bulk movement ofthe sample around the electrodes.

Whilst the membrane 42 allows the diffusion 44 of the measurandchemical, in this case NH₄ ⁺, towards the sensing volume 46, it stopssample movement due to liquid currents from dispersing the pH-shiftedvolume near the electrodes. The membrane 42 could be hydrophilic orhydrophobic, and of varying porosity. In a preferred embodiment, themembrane is hydrophilic on the surface facing away from the sensor 36,and has a low adhesion for proteins. These characteristics will reducebio-fouling, prolonging service intervals by reducing the need forcleaning or replacement of the membrane 42.

The required electrolytic pH shifting must take place in ‘clean’ water.The electrodes 38, 40 are housed behind semi-permeable membrane 42 toprevent the incursion of solids into the sensing volume 46 or onto theelectrodes.

In operation, the measurand chemical 44, in this case NH₄ ⁺, diffusesinto the sample volume 45 near the upper, central electrode 40, which inthis example is operated as the cathode. The excess of electrons at thecathode causes excess hydroxyl ions to be generated in the neighbouringregion by the following reaction:2H₂O+2e ⁻→2OH⁻+H₂↑, or2H₂O+O₂+4e ⁻→4OH⁻

Either of these reactions will cause an increase in OH⁻ ions, leading toa reduced concentration of H⁺ ions and an accordingly raised pH. Due tothis raised pH and increased concentration of OH⁻ ions in the liquidaround the cathode 40, some of the dissolved ammonium ions NH₄ ⁺ in theliquid around the cathode return to their gaseous ammonia form NH₃:NH₄ ⁺+OH⁻→H₂O+NH₃↑.

The generated ammonia gas evaporates 34 into a gas volume 48 underneaththe gas sensor 36, where it can be detected. The volume of ammoniaproduced represents the concentration of ammonium ions in the originalsample, since the volume of liquid under test will not change, beingfixed by the sample volume 45.

A small gas volume 48 is preferably maintained in front of the gassensor 36 to allow the measured gas to diffuse into contact with as muchof the surface of the sensor as possible, and for the density (partialpressure) of the gas of interest to be as constant as possible over thesurface of the sensor.

The gas volume 48 is preferably enclosed by a porous, gas-permeable ring50, which allows the build-up of measurand gas, in this case ammonia, toa more concentrated level. By adjusting this porosity of the ring 50,the sensitivity and response time of the overall sensor equipment may beadjusted. In some cases the porous ring may not be needed, for examplein high concentration applications, where sufficient gas is emitted forsuch collection to be unnecessary. A gas permeable membrane 60 ispreferably provided, to delimit the gas volume 48 from the sample volume45, and to prevent fouling of the gas sensor 36 by the liquid 32. Thismembrane may not be necessary, according to the type of detector 36used.

A substrate 52 is provided to support the electrodes. It must provide atransverse passage at least for ions of the species to be measured, andpreferably also for the liquid in which they are dissolved. This may beachieved by providing a substrate 52 which is porous or permeable, forexample having an array of small holes in it, particularly in thecentral portion 54 to allow measurand species, in this case NH₄ ⁺, topass through. The substrate 52 should preferably be as thin as possibleto improve sensor response time by minimising path length between theelectrodes while keeping them separated. The substrate should also bechemically inert and resistant to extremes of acid or alkali, since asignificant pH differential may exist between its two sides.

While the measurand species needs to be able to pass through thesubstrate, the passage should be as restricted as possible, to reducethe recombination between H⁺ and OH⁻ ions, which would otherwise cancelthe generation of alkaline and acidic regions around the electrodes andaccordingly prevent efficient detection of the measurand species such asammonia.

The upper electrode 40 must be kept in closer electrical contact withthe sample liquid 45 than with the lower electrode 38, in order that theions generated around the upper electrode serve primarily to adjust thepH of the sample volume, rather than to pass an electric current betweenthe two electrodes. The volume 45 around the upper electrode should bekept small, to allow a relatively large pH shift to be obtained. Bymaking the volume around the other electrode 38 somewhat larger, the pHshift of the lower region will be lower, and the tendency for thegenerated H⁺ and OH⁻ ions to recombine will be reduced.

The electrode 40 and the volume 45 should each be quite thin, but offairly large area. This allows a high ratio of electrode surface area tosample volume 45, in turn providing for a relatively large pH shift andeffective ammonia release while maintaining a sufficient sample volume45.

Selection of the type and form of electrodes may have a significantinfluence of the effectiveness of the detector as described. It has beenfound that electrochemical activity preferentially takes place at theparts of the electrodes closest to each other. By providing electrodesin the form of two parallel planar meshes, the surface area of theelectrodes are increased due to the mesh form, and the maximum possibleuse is made of the electrodes by placing them with as much surface areaas possible in a “nearest” position. This also ensures minimumresistance to current flowing between the electrodes.

The electrodes must be robust enough to withstand bubble formation, andmust be resistant to chemical attack. They may, for example, be madefrom a noble metal, or carbon. In certain embodiments, they may beprinted onto a porous, or perforated, alumina substrate.

There is a wide variety of electrodes that are suitable for use with thepresent invention. The electrodes may be simple planar units ofconductive material. They may comprise a metal, carbon, a semiconductor(such as a metal oxide), a conducting polymer, or a composite of two ormore of these materials. We have found that electrodes comprisingvitreous carbon, platinum, gold or stainless steel are suitable;however, other metals, such as silver may be used instead. Composites oftwo metals, such as platinum coated on titanium may be used. Theelectrodes may comprise solid, woven, porous or thick- or thin-filmprinted electrodes.

Electrodes comprising a piece of metal gauze have been found effective.

Gaps, or porosity of the material 50, must be provided sufficient toallow the generated ammonia gas to escape from gas volume 48 afterdetection, and to allow fresh liquid 32 or measurand species to enterthe sample volume 45. By controlling the size of such gaps, or theporosity of material 50, the sensitivity of the device may be adjusted.

The inside of the housing 62 and material 50 must be chemically robustenough to resist the formation of sulphuric and hydrochloric acid (amongothers). This is a typical natural consequence of the elimination ofammonia from polluted water.

A heater, schematically represented at 56, is included on the substrateto control the temperature of the liquid within the sensing volume.Ammonia, along with certain other species of interest, becomes lesssoluble at certain temperatures, as well as certain ranges of pH. Thevapour pressure of ammonia is quite low below 10° C., which temperatureis common in natural and waste water supplies. By controlling both thepH and the temperature of the solution to achieve conditions of reducedsolubility, a maximum proportion of the available ammonium ions will beconverted to ammonia gas, enabling greater accuracy of measurement. Byheating the liquid within the sensing volume to a controlled temperaturesignificantly above the ambient liquid temperature, such as 45-60° C.,the operating temperature will not be affected by fluctuations in theambient temperature of the liquid under test. Heater 56 will heat theliquid in the measurement volume, and also the sample preparation unitof the present invention. This increased temperature should also reduceor eliminate bio-fouling on the membranes 42, 60, particularly if theoperating temperature is selected with this in mind. A temperaturesensor, not shown in the drawing, and associated control circuitry, ispreferably also provided, to allow accurate control of the temperatureof the solution, particularly in the sample volume 45 around and aboveelectrode 40. The heater 56 may be provided only in the sample volume45, if desired, but a more extensive heater will be able to heat alarger proportion of the solution above membrane 42, which should leadto a more stable temperature, and may help to prevent condensation ofvapour from the sample 45 as it leaves the device.

Heating of the liquid contributes to keeping the membrane(s) 42, 60clear of bio-fouling. The high temperature and the acidic/alkalineenvironment is not suitable for biological growth.

By maintaining a constant temperature of the measured liquid, allmeasurements are known to be comparable, being taken under identicalconditions of temperature. The temperature-dependent solubility ofammonia, for example, is rendered constant by measurement at constanttemperature.

In some applications it may be necessary to place one of the electrodesoutside the membrane, directly in the liquid 32. This could increase thepH shift inside the sample volume 45, partly by impeding therecombination of the H⁺ and OH⁻ ions by increased physical separation.

In the example illustrated in FIG. 3, the anode 38 could be placedoutside the membrane 42, although it would then be subject to anincreased likelihood of bio-fouling or accumulation of solids. Thecathode 40 would remain within the membrane 42, where it can generatethe required ammonia gas in proximity to the gas sensor 36.

The entire assembly of electrodes 38, 40, substrate 52, heater 56,membranes 42, 60 and sensor 36 may be enclosed within an outer housing62. This housing could be formed of a solid sheet, which would preventdiffusion of liquid or the species under examination into themeasurement volume 45 through such outer housing. Alternatively, asemipermeable/porous membrane may be used instead, which would allowsome in-diffusion of the liquid or ions under test through the outerhousing 62.

Certain parts of the structure, for example the outer housing 62, themembrane 42 and the substrate 52, may need to be strengthened if used ina high pressure environment, or where a high pressure differentialexists between the various parts of the test equipment. Such conditionsmay exist, for example, where the sensor is to be used in a water supplyline, or within an effluent pipe.

The overall diameter of the unit will be limited by the ability to shiftthe pH of the sample and by the size of the gas sensor. The ability toshift pH will, in turn, depend on the nature of the electrodes, thepotential difference applied between the electrodes, and the volume ofliquid in the sensing region.

In an alternative embodiment, the gas sensor 36 may be replaced by anion selective electrode (ISE) placed in one of the pH shifted regions,in order to detect the measurand species. Some species of interest maybe more amenable to liquid phase sensing, and such an ISE could providemore sensitive or more accurate detection. Ion sensitive electrodesensing requires the provision of a reference electrode and an ionsensitive electrode, which will produce an output representative of theconcentration of the ion detected, such as NH₄ ⁺ or H⁺.

Other sensors can be used instead. For example, other optical orelectrical sensors may be used, and they may be invasive ornon-invasive.

While the above-described embodiment creates an alkaline environmentaround the working electrode 40, other species of interest may requirean acidic environment in order to return to their gaseous state. In suchcases, the polarity of the potential difference applied between theelectrodes is reversed. Electrode 40 becomes the anode, and createshydrogen ions H⁺ by the following reaction:2H₂O−4e ⁻→O₂↑+4H⁺.

This provides a region of increased density of H⁺ ions, that is, anacidic zone, in the measurement volume. When the appropriate pH andtemperature conditions are reached, the species of interest returns toits gaseous phase, and may be detected by a suitable detector 36.Alternatively, a liquid phase sensor such as an ion sensitive electrode(ISE) may be used to detect the species of interest while still in thedissolved state. When used in this way to detect a basic species, thesample surrounding the electrode 40 and next to the sample 45 willbecome acidic. This should help to keep the electrode 40 and membrane 60clean.

It may be necessary to measure the pH in the sample volume 45 to achieveoptimum control of pH. In certain embodiments of the present invention,the conductivity or other properties of the sample in the sample volume45 could be measured to control the potential difference applied betweenthe electrodes to achieve better pH control in the measurement volume.

In certain embodiments of the present invention, the electrode shapescould be varied to optimise pH differentiation between the basic regionaround the cathode and the acidic region around the anode.

The electrodes may comprise metal film on ceramic; alternatively, theymay be composed of gauze structures with solid or gauze insulatingsubstrate layer in between.

The housing 62 needs to be constructed of a suitable material whichshould be resistant to fouling and also resistant to acidic and alkalineenvironments generated inside the unit. It should also be robust enoughfor installation in activated sludge aeration ponds.

Some active monitoring and control might be required, since no singlecurrent/voltage will be ideal for all liquids or species of interest.

FIGS. 4A-4F illustrate steps in the manufacture of an integrated elementaccording to an aspect of the present invention. The integrated elementincludes the substrate 52, the heater 56, if any, the electrodes 38 and40 and the necessary protective coatings and electrical connections toallow the integrated element to operate as a part of the detectionapparatus according to the present invention.

FIG. 4A shows a substrate 52 formed in a first step in the manufactureof the integrated element. The substrate comprises a circular disc of96% alumina ceramic, of 635 μm thickness and 66.2 mm diameter. One ormore flats 70 may be provided around the circumference of the substrate,to assist in orientation of the substrate during subsequent processing.An array of through holes are formed in a central region of thesubstrate. These holes may be 1 mm in diameter arranged on a squarematrix at 2 mm pitch. Provision may also be made at this stage for laterelectrical connections to the various electrical components of theintegrated element. For example, surface mount connection point 73 maybe provided for electrical connection to features on the upper surfaceof the substrate, together with a through-plated holes 74 to provideelectrical connection to the lower surface. The through hole andconnection points may be at 2 mm pitch, offset from the edge of thesubstrate by 7 mm, and be gold plated. The integrated element may, ofcourse, be produced to other dimensions, according to the materials usedand the type of testing to be carried out.

As shown in FIG. 4B, heater element 56 may next be provided on the lowersurface of the substrate, for example by screen printing a resistivepaste. The heating element may, for example, have a resistance of about25Ω, and be of uniform profile. The terminals of the heater are providedin electrical contact with two of the through holes 74. As illustrated,the heater element preferably comprises a dual serpentine structure,arranged partially between the holes 72 and covering the majority of thelower surface of the substrate, in a circular region of about 54 mmdiameter, concentric with the substrate itself.

The use of platinum electrodes printed onto opposite sides of a ceramicsubstrate has been found to be effective, robust and relativelyeconomical.

As shown in FIG. 4C, a chemically resistant waterproof insulating layer76 is next applied over the heater. This layer must be resistant to acidor alkali and must prevent the liquid under test from reaching theheater 56. A cermet insulator may be applied by screen printing.Preferably, the insulator 76 is double-printed to reduce the chance ofpinholes allowing the liquid under test to reach the heater 56. Theinsulator preferably overlaps slightly on all sides of the heater.

FIG. 4D shows the electrode 38, in this case made of platinum byscreen-printing platinum-rich paste and heating. The platinum electrodeis formed in electrical contact with one of the through-plated holes 74.The platinum electrode is preferably annular, concentric with thesubstrate. The electrode may have an outside diameter of about 52 mm andan inside diameter of about 43 mm. Other materials and methods could, ofcourse, be used to provide the electrode 38.

FIG. 4E shows a view of the upper surface of the integrated element,showing the formation of the electrode 40. This electrode may befabricated with identical materials and methods to those used to producethe electrode 38. For example, platinum-rich paste may be screen printedonto the substrate and baked. The electrode must be in electricalconnection with a contact 73. Preferably, the electrode 40 is providedaround and between the holes 72, but with a certain clearance to ensurethat the paste does not foul the holes. A clearance of 0.25 mm may besufficient. In this example, the electrode 40 may have an outer diameterof 30 mm.

FIG. 5 shows a cross-section through the integrated element 80 of FIG.4E, along the line V-V. Features corresponding to those illustrated inearlier drawings carry corresponding reference numerals. Contact pins 79may be provided, in respective contact with the pads and through-holes73, 74, to facilitate electrical connection to the electrodes 38, 40 andthe heater 56. The integrated element may accordingly be seen to embodynumerous features of the structure of the invention illustrated in FIG.3.

FIG. 6 shows an example of a practical detection device incorporatingthe present invention. Outer casing 62, preferably of stainless steel,nylon or another material which is mechanically strong but resistant tocorrosion, carries a threaded ring 82 against which a threaded end piece84 may be tightened. Semi-permeable membrane 42, which is preferablyhydrophilic, is placed across the open end of casing 62 and may bepulled tight by action of o-ring 86 when end-piece 84 is tightened.Preferably, threaded ring 82 is rotated while end-piece 84 is heldstationary. This allows the membrane 42 to be tightened withoutimparting any rotational forces to it. An integrated element 80, such asthat shown in FIGS. 4-5, is provided, isolated from the liquid undertest 32 by the semipermeable membrane 42. The membrane 42 is spaced awayfrom the assembly 80 by the thickness of a gasket 88 which is broughtinto compression by the action of end-piece 84 when tightened. End-piece84 has an open window 90 allowing access by the liquid 32 to themembrane 42. Preferably, the edges 92 of the window are chamfered toreduce the likelihood of a build-up of bio-fouling or solid matter, andto promote free circulation of the liquid 32 in the vicinity of themembrane 42. Liquid is able to traverse the integrated element 80 andenter the sample volume 45. Gas permeable membrane 60 allows access bythe ammonia, or other gas produced as appropriate, into the gas volume48, which is at least partly exposed to sensor 36. The gas permeablemembrane 60 preferably comprises a hydrophobic, protein resistantmembrane, for example it may comprise a porous Teflon™ hydrophobicmembrane. The membrane is preferably protein resistant to reduce orprevent the build-up of bio-fouling on the membrane. A porous ring 50 ispreferably provided, to retain the measurand gas within the space 48,while allowing it to out-diffuse with time through vents 96. Sensor 36detects the measurand species, such as ammonia, and produces acorresponding output according to the type of sensor used. Electricalcontacts 79 affixed to the integrated element 80 provide electricalconnection to the electrodes 38, 40 and any heater 56 present on theintegrated element 80. The device is also preferably heated by a heater100 to prevent condensation of water vapour and other gases in the bodyof the sample preparation device. A block 61 retains the heater and thevarious components in place. It contains a cavity defining the samplevolumes 48, 45. The block should be of a corrosion resistant material.In order to assist in maintaining a constant temperature, the block maybe of a material with a high thermal coefficient. For example, the blockmay be of brass, stainless steel or a plastic such aspolyethyletherketone (“peek”). The block may be formed in two or moreparts, for example, joined along line J-J, to facilitate manufacture andassembly operations. A body 94 closes the housing 62, but allows egressof gas through vents 96. The body 94 houses various electroniccomponents required for the control and measurement functions; and alsocontains a power supply to provide power to the sensor 36, theelectrodes and the electronics in body 94. O-rings 97 located in grooves98 in the block 61 may be provided, sealing between the integratedelement 80 and the block 61, preventing ingress of the measurand fluid32 other than by way of the membrane 42.

In a certain embodiment, the integrated element 80 may have a diameterof 66.2 mm, and the entire structure shown in FIG. 6 may have a diameterof 110 mm. In this case, the housing 62 may have an inside diameter of82 mm and an outside diameter of 88 mm.

FIG. 7 shows a view of the device of FIG. 6 in the direction of arrowVII. Dotted line 99 shows the region of the integrated element 80 whichis exposed to the sample volume 45. The other features bear referencenumerals corresponding to the reference numerals employed in thepreceding drawings.

The present invention accordingly provides a sample preparation deviceand method for detection of a broad range of chemicals, particularlythose which become volatile under certain conditions of pH andtemperature. Some examples include ammonium ions (NH₄ ⁺) and ammonia(NH₃), Chlorine (Cl), Cyanide (CN⁻) and mono-, di-, and tri-chloramides(NH₂Cl, NHCl₂, NCl₃).

In one embodiment, the gas sensor may be adapted to measure carbondioxide gas. This provides a means to measure the total inorganic carbonlevel of an aqueous sample. The acidification of the sample liberatescarbon dioxide from any bicarbonates and carbonates present in thesample.

In another embodiment, the gas sensor may instead be adapted to measuresulphur dioxide gas. Acidification of the sample (to a pH below 0.7)enables the “free” sulphur dioxide levels to be monitored; this isespecially useful in the food and drink industries. Samples containing“bound” sulphur dioxide could be analysed by increasing the pH to avalue above 12, when sulphur dioxide will be liberated and can bemeasured. These two techniques could be combined to provide a measure ofthe total sulphur dioxide levels; both measurements could be madesimultaneously with the present invention.

The invention finds application in monitoring water treatment processes,or measuring concentrations of certain chemicals in industrial effluent,or monitoring chemicals in water supplies.

The present invention accordingly provides, in certain preferredembodiments, an assembly including a hydrophilic membrane to protect theelectrodes from suspended solids in the sample, a thin electrodeassembly to achieve a faster response and the addition of a heater fortemperature control to achieve consistent detection conditions andimproved anti-fouling properties.

By way of examples only, the following materials could be used for thehydrophobic and hydrophilic membranes, although these examples are notlimiting of the invention in any way, and other materials may be usedfor the membranes, as appropriate. The hydrophobic membrane may beformed from commonly available hydrophobic PTFE(polytetrafluoroethylene) membrane. Similarly, the hydrophilic membranemay be formed from hydrophilic PTFE. In a specific example, thefollowing material may be used. Certain characteristic properties of thematerial are also listed.

Filter Medium: Hydrophilic polysulfone Pore Size: 0.2 μm to 0.45 μmTypical Thickness: 165 μm Typical Water Flow for pore size 0.2 μm: 12mL/min/cm² at 0.7 bar Rate: for pore size 0.45 μm: 50 mL/min/cm² at 0.7bar Maximum Operating 121° C. Temperature-Water: Extractables-Boiling<3.5% Water: Minimum Bubble for pore size 0.2 μm: 2.5 bar Point-Water:for pore size 0.45 μm: 1.5 bar Biological Safety: Passes USP ClassVI-121° C. Plastics Tests

While the present invention has been particularly described withreference to the detection of dissolved species in water, it may beapplied to the detection of species dissolved in other fluids, in whichcase references to “water”, “waterproof” “hydro-” etc. should beconstrued as appropriate.

The invention claimed is:
 1. An integrated element comprising: a porousor permeable substrate (52); a first electrode (40) formed on a firstside of the substrate (52); a second electrode (38) formed on a secondside of the substrate (52) a barrier (50) around the first electrode,for restricting the movement of a liquid under test; and a gas volume(48) provided between the, first electrode (40) and a sensor (36);wherein the substrate (52) comprises physical barrier (52) between thefirst and second electrodes, wherein the physical barrier is porous orpermeable to a liquid, under test, further comprising an ion sensitiveelectrode as the sensor (36).
 2. An integrated element according toclaim 1 wherein the integrated element further comprises a heaterelement (56) located on the substrate, the heater element beinginsulated (76), wherein the electrodes and the heater element are formedby deposition of respective conductive layers onto the substrate andfurther wherein the firs and second electrode are planar and concentric,the first electrode being substantially circular, the second electrodebeing substantially annular having an inside diameter larger than thediameter of the first electrode.
 3. An integrated element according toclaim 2, further comprising a second heater (100) for preventingcondensation of vapour generated from a liquid under test.
 4. Anintegrated element according to claim 1 wherein a gas permeable membrane(60) is provided to prevent a liquid in the sample volume from enteringthe gas volume, while allowing any gas emanating from the liquid toenter the gas volume, wherein the pas permeable membrane (60)ishydrophobic.
 5. An integrated element according to claim 1 and furthercomprising a selective gas detector as the sensor.
 6. An integratedelement according to claim 1 wherein a semipermeable membrane (42) ishydrophilic, at least on the side which is directed away from a samplevolume.
 7. An integrated element according to claim 1, comprising a pHcontroller and a temperature controller.
 8. An integrated elementaccording to claim 1 further comprising an outer housing (62) to containthe electrodes and substrate and further to contain a heater, a membraneand a sensor.
 9. A method of preparing a sample of liquid for detectionof impurities, comprising the steps of: providing first and secondelectrodes within the liquid; applying a voltage between the first andsecond electrodes, to thereby create a region of increased pH and aregion of reduced pH at the respective cathode and anode electrodes;detecting a characteristic of the liquid in a first of the regions ofchanged pH, characterized in that the method further comprises the stepsof: passing liquid from a body of liquid under test through asemipermeable membrane to reach the first region, thereby preventingsolids carried in the body of liquid from reaching the first region,while allowing the impurity to be detected to reach the first region;and heating the liquid in the first region of changed pH to a constanttemperature.
 10. A method according to claim 9 wherein the temperatureof the liquid in the first region is adjusted to promote detection ofthe impurity to be detected.
 11. A method according to claim 9 whereinthe temperature of the liquid in the first region is adjusted to deterthe formation of biofouling on the membrane.
 12. A method according toclaim 9, further comprising: in response to the alteration of the pH ofthe first region, converting the impurity to be detected into a gaseousform, and detecting the impurity with a suitable gas detector.